One of the critical points in using a hairspring in a high-precision clock movement is the reliability of the attachments (in settings) of the hairspring to the balance staff and to the balance bridge. In particular, the attachment of the hairspring to the balance staff is usually performed using a collet, which originally was a small split cylinder intended to be driven onto the balance staff and drilled laterally to receive the interior end of the actual hairspring proper. The development of micromanufacturing techniques, such as DRIE methods for silicon, quartz and diamond or UV-liga methods for Ni and NiP, have opened up options regarding the shapes and geometries used.
Silicon is a very advantageous material from which to make clock springs and micromanufacturing techniques allow the collet to be produced such that it is integral and manufactured as one with the hairspring. One potential problem is that silicon does not have a plastic deformation domain. The collet may thus soon break if the stresses exceed the maximum permissible stress and/or the elastic limit of the material. It is therefore necessary to be sure to dimension the collet both to hold the hairspring on the balance staff when the oscillator is operating (minimal tightening torque) and also so that the collet can be assembled with staffs the diameters of which may fluctuate, all this without breaking or suffering plastic deformation if the diameter of the balance staff remains within a given tolerance band.
Thus, there are various documents that disclose collet geometries.
European patent application published under no. EP 1 826 634 proposes, in its FIG. 4 in conjunction with line 34 of column 3, a collet comprising elastic zones consisting of curved arms. That document does not indicate where the hairspring is to be fixed.
European patent applications published under numbers EP 1 513 029 and EP 2 003 523 propose collets having a triangular opening. The hairspring is fixed in place at an attachment point (reference 3 in the figures of both documents) located at one of the vertexes of the triangles. The collet is formed of an external stiffening structure to which are attached flexible arms which deform to accommodate the balance staff.
European patent application published under no. EP 1 655 642 describes in its FIG. 10D a hairspring of a hairspring resonator having a collet the opening of which is circular. In this case, the balance is attached using rounded arms.
Also, patent application WO2011026275 discloses a hairspring-collet assembly with a collet having a bore provided with four circular bearing parts to receive the balance staff. The bearing parts are delimited by longitudinal grooves made in the bore of the collet.
The geometries described in these documents are not entirely satisfactory which means that many hairsprings (made of silicon, diamond, quartz, etc.) mounted on movements are equipped with a conventional collet which is then driven onto and/or bonded to the balance staff.